With a reduction in the sizes of electric and electronic apparatuses in recent years, a reduction in the size and an increase in the efficiency of a magnetic material are required. As a magnetic material according to the related art, for example, in a choke coil used for a high frequency circuit, there are a ferrite core using ferrite powder and a dust core which is a compact of metal magnetic powder.
Among them, the ferrite core has disadvantages in that the saturation magnetic flux density is low and DC bias characteristics are low. Therefore, in the ferrite core according to the related art, in order to ensure the DC bias characteristics, a gap of several hundreds of micrometers is provided in a direction perpendicular to the flux path, thereby preventing a reduction in an inductance value L during DC bias. However, this wide gap becomes a source of generating buzzing noise, and leakage magnetic flux generated from the gap causes a significant increase in copper loss of a winding wire particularly in a high frequency band.
Contrary to this, the dust core produced by compacting metal magnetic powder has a much higher saturation magnetic flux density than that of the ferrite core, and therefore it can be said that the dust core is advantageous for size reduction. In addition, unlike the ferrite core, the dust core can be used without a gap, and thus has low buzzing noise and low copper loss due to leakage magnetic flux.
However, it cannot be said that the dust core is better than the ferrite core in terms of magnetic permeability and core loss. In particular, in the dust core used in a choke coil or an inductor, the temperature of the core increases with an increase in the core loss, so that it is difficult to achieve a reduction in the size. In addition, in order to enhance magnetic characteristics of the dust core, the compacting density thereof needs to be increased, and during production, a compacting pressure of equal to or higher than 5 ton/cm2 is generally needed and a compacting pressure of equal to or higher than 10 ton/cm2 is needed depending on products.
Here, the core loss of the dust core is generally caused by hysteresis loss and eddy-current loss. In a metal material, due to a low specific resistivity, with regard to a change in magnetic field, eddy current flows so as to suppress the change, so that the eddy-current loss becomes a problem. The eddy-current loss increases in proportion to the square of the frequency and the square of the area through which the eddy current flows. Therefore, by coating the surface of the metal magnetic power with an insulating material, the area through which the eddy current flows is suppressed only to the inside of the metal magnetic powder particles from the entire core across the metal magnetic powder particles, so that the eddy-current loss can be reduced.
On the other hand, since the dust core is compacted at a high pressure, a large amount of processing strain is introduced to the magnetic material, so that magnetic permeability is reduced, and so that hysteresis loss is increased.
In order to avoid this, after compacting, a heat treatment for relieving the strain is performed as needed. In general, in a metal material, recovery is a phenomenon that occurs at a temperature of equal to or higher than half the melting point, and in order to sufficiently relieve the strain in an alloy with an Fe-rich composition, a heat treatment needs to be performed at least 600° C. or higher, and preferably, at 700° C. or higher.
That is, in the dust core, it is important to realize a high-temperature heat treatment in a state where insulating properties are ensured in the metal magnetic powder.
However, most of the organic resins, such as an epoxy resin, phenolic resin, or vinyl chloride resin, used as an insulating binder of the dust core according to the related art have low heat resistance and undergo thermal decomposition when a high-temperature heat treatment is performed thereon in order to relieve the strain, and thus cannot be used.
As a solution of this problem, for example, like Patent Document 1, a technique of using a polysiloxane resin is proposed.
However, even in this technique, the heat resistant temperature is about 500 to 600° C., and a heat treatment at a higher temperature is difficult.
In order to solve the problems, the present invention provides a composite magnetic material which enables a high-temperature heat treatment, thereby realizing excellent magnetic characteristics.